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Abstract:

A composition and method for cleaning turbine engine components (10)
during servicing. An embodiment of the invention includes a colloidal
mixture or slurry (22) of nanoparticles. The slurry may be nontoxic and
provide optimal cleaning of tiny surface-exposed crevices (18) of braze
joints and components. When a colloidal mixture is in a polar solvent,
the pH of the slurry is maintained at about 5 to 9 and at the isoelectric
point of the nanoparticles to minimize or prevent agglomeration. When a
colloidal mixture is in a nonpolar solvent, the pH of the slurry is
maintained at about 5 to 9 and at the isoelectric point of the
nanoparticles to minimize or prevent agglomeration by use of surfactant
additives.

Claims:

1. A method of repairing a turbine engine component, the method
comprising: applying a colloidal solution to a surface of a turbine
engine component having a surface opening crevice, the colloidal solution
comprising nanoparticles suspended in a solvent; allowing the colloidal
solution to penetrate the crevice and loosen a contaminant material
disposed within a tip region of the crevice; removing the colloidal
solution and loosened contaminant material from the crevice and surface;
and depositing a repair material onto the surface and into the crevice to
penetrate the tip region of the crevice previously occupied by the
contaminant material.

2. The method of claim 1, further comprising agitating the colloidal
solution against the contaminant material within the crevice with
ultrasonic energy.

3. The method of claim 1, wherein the solvent is polar and the pH of the
solution is maintained between 5 and 9 at an isoelectric point of the
nanoparticles.

4. The method of claim 1, wherein a hardness of a material of the
nanoparticles is selected to be harder than a hardness of the contaminant
material but softer than a hardness of a material of the surface.

5. The method of claim 1, further comprising selecting the nanoparticles
from the group consisting of ceramics, metal oxides, carbides, nitrides,
and metalloids and combinations thereof, wherein said composition has a
pH of about 5 to 9.

6. The method of claim 1, further comprising the solution to comprise the
formula, when in polar solvent, nano silica+H2O+PAN PMMA (Particle
D(50)=20, 50, and 80 nm, SSA=130 to 35 m2/g, 5 to 25 v/o solids).

7. The method of claim 1, further comprising the solution to comprise the
formula, when in polar solvent, nano alumina+H2O+PAN PMMA (Particle
D(50)=20, 50, and 80 nm, SSA=130 to 35 m2/g, 5 to 25 v/o solids).

8. The method of claim 1, further comprising the solution to comprise the
formula, when in polar solvent, nano zirconia+H2O+PAN PMMA (Particle
D(50)'=20, 50, and 80 nm, SSA=130 to 35 m2/g, 5 to 25 v/o solids).

9. The method of claim 1, further comprising the solution to comprise the
formula, when in nonpolar solvent, nano silicon
carbide+Decalin/Hexane+PVC (Particle D(50)=20, 50, and 80 nm, SSA=130 to
35 m2/g, 5 to 25 v/o solids)

10. The method of claim 1, further comprising the solution to comprise
the formula, when in nonpolar solvent, nano silicon
nitride+Decalin/Hexane+PVC (Particle D(50)=20, 50, and 80 nm, SSA=130 to
35 m2/g, 5 to 25 v/o solids).

11. The method of claim 1, further comprising selecting the nanoparticles
to comprise two different materials.

12. The method of claim 1, further comprising selecting the solution to
exhibit a Zeta potential of at least +/-20 mV.

Description:

FIELD OF THE INVENTION

[0001] The invention generally relates to turbine engine servicing, and
more particularly to a composition and method of cleaning and repairing
turbine engine components having surfaces containing small cracks or
crevices.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] Gas turbine engine components may be formed of superalloy material
known for high temperature performance in terms of tensile strength,
creep resistance, oxidation resistance, and corrosion resistance. The
superalloy component may be a nickel-base alloy, wherein nickel is the
single greatest element in the superalloy by weight. Illustrative
nickel-base superalloys include at least about 40 wt % Ni, and at least
one component from the group including cobalt, chromium, aluminum,
tungsten, molybdenum, titanium, and iron.

[0003] Various turbine engine components crack, erode or experience
conditions necessitating a repair. No joining process (braze, bond, weld,
etc) will have a good result with contaminants/oxides present; e.g.
brazes won't adhere, welds will have defects etc. In situations where the
contaminant or oxide cannot be removed by a simple mechanical means due
to its location in a crevice or crack, special cleaning techniques are
required. Note that the term contaminant may be used herein to include
both oxides and non-oxides, although oxide contaminants are commonly
found in gas turbine applications.

[0004] Prior to servicing a turbine component, it is necessary that
contaminants/oxides be removed so that a subsequent braze, for example,
will adhere to the base material. A fluoride ion cleaning (FIC) procedure
currently known in the industry utilizes hydrofluoric acid (HF) at
elevated temperatures converting metal oxides to gaseous metal fluorides
and water. Because hydrofluoric acid is an extremely corrosive acid, it
is known that the acid may impede servicing by degrading an existing base
material by depleting compositional elements and/or causing intergranular
attack. Furthermore, hydrofluoric acid is extremely dangerous to handle
and may cause skin injury or corneal damage. U.S. Pat. No. 7,303,112
describes a method of repairing a braze joint which includes the use of
both an alkali metal molten salt bath and an acid solution.

[0005] Thus, there is an ongoing need for an improved turbine component
repair procedure incorporating a safe and effective cleaning process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The invention is explained in the following description in view of
the drawings that show:

[0008] FIG. 2 is a schematic cross-sectional view of a prior art component
exhibiting a surface opening crevice filled with a contaminant material.

[0009] FIG. 3 is the component of FIG. 2 after a prior art cleaning/repair
process.

[0010] FIG. 4 is the component of FIG. 2 undergoing a cleaning step as
part of a repair process in accordance with one embodiment of the present
invention.

[0011]FIG. 5 is the component of FIG. 4 upon completion of the repair
process.

[0012] FIG. 6 block diagram showing an embodiment of steps for a repair
process as described herein.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The present inventors have found that current gas turbine component
repair procedures are sometimes less effective than desired. FIG. 1 is an
illustration of a gas turbine engine component 2 prior to repair
exhibiting surface opening crevices in the form of service induced
cracking 4. It is known that surfaces must be clean prior to receiving a
material deposition repair such as a braze, and that incomplete cleaning
of the surface can result in an unacceptable braze result. However, even
when the surface is cleaned in accordance with the prior art acid
cleaning procedure, the braze will not adhere to an oxide and 100% fill
of joint cannot be obtained. The operating life of the repaired region
will not be completely restored.

[0014] The present inventors have discovered that the prior art acid
cleaning procedure does not always completely remove all
contaminants/oxides from tiny crevices that may exist in a surface to be
repaired, even after a thorough fluoride ion cleaning. FIG. 2 is a
schematic cross-sectional illustrate of a component 10 exhibiting a
surface opening crevice 12 disposed along a surface 14 to be repaired.
The surface 14 exhibits a layer of a service induced contaminant 16 which
extends into the crevice 12 and fills a tip region 18 of the crevice.
Shown is a 16 continuous oxide layer; however, oxide may not always be
continuous across 14 base material. FIG. 3 illustrates that same
component 10 after a repair procedure wherein the surface 14 has been
exposed to a prior art acid cleaning process and then a repair layer of
braze material 20 has been applied to the surface. Note that the braze
material 20 does not penetrate into the tip region 18 because the
contaminant material 16 has not been removed from the tip region 18. This
leaves the tip region 18 of the crevice 12 as a stress riser during
subsequent machine operation. The existence of the unbrazed subsurface
crevice tip region 18 and the resulting stress concentration facilitates
the growth of a new crack in the repaired surface in a time period that
is less than would have been required for a crack to form in an
equivalent completely solid region of the component. Thus, the present
inventors have developed a novel repair procedure which incorporates
cleaning steps specifically targeting the removal of contaminants/oxides
from within surface-opening crevices, thereby solving this previously
unappreciated problem of the prior art cleaning/repair procedures.

[0015] FIG. 4 illustrates component 10 undergoing a cleaning step as part
of an embodiment of the present invention wherein a colloidal cleaning
solution 22 is applied to the surface 14 and layer of contaminant 16, as
more fully described below. As a result, the contaminant 16 is removed
from the crevice tip region 18, so that when a layer of repair material
such as braze material 20' is applied, as illustrated in FIG. 5, the
braze material 20' extends into the tip region 18 previously occupied by
the contaminant material. This minimizes or eliminates any stress
concentration during subsequent operation of the component 10 and allows
the component to achieve a repaired life expectancy approaching that of
its new condition life expectancy.

[0016] FIG. 6 illustrates the steps of a repair procedure 30 incorporating
an embodiment of the present invention. A component, such as a gas
turbine engine part, is removed from service at step 32. A surface of the
component needing repair and exhibiting a surface opening crevice is
prepared for repair by applying a colloidal cleaning solution at step 34.
At step 36 the solution is allowed to penetrate into the crevice and the
chemical-mechanical action of the solution is allowed to loosen the
contaminant contained within the crevice. Optionally, mechanical energy
such as ultrasonic energy may be applied to the solution within the
crevice at step 38 to enhance the cleaning action within the crevice. The
loosened contaminant is then removed from the crevice and the surface at
step 40 such as by vacuuming. A layer of repair material is applied at
step 42, with the repair material now penetrating the crevice to occupy a
tip region of the crevice that was previously occupied by the contaminant
material. Upon completion of the repair procedure, the component is
returned to service at step 44.

[0017] The instant invention incorporates a composition and method for
cleaning surfaces such as turbine engine components and braze joints.
More specifically, solutions are customized to target and remove specific
oxide deposits embedded in narrow surface-opening cracks prior to a braze
application. Solutions that may be considered include a colloidal mixture
or slurry of nanoparticles in a solvent wherein the concentration of
nanoparticles is about 0.5 wt % to about 70 wt %. Terms nanoparticle
solution or colloid may be used to describe the solid-liquid mixtures,
all of which contain distinct nanoparticles dispersed to various degrees
in a medium. The slurry may preferably be nontoxic and provide optimal
cleaning of tiny crevices existing in braze joints and other portions of
gas turbine engine components. Colloid cleaners are known for the
cleaning of a variety of types of surfaces, such as walls, floors,
machinery, carpet, etc., and they function by breaking surface tension
and holding grease, oil and dirt in suspension, thus making them easier
to remove from a surface. However, the present inventors have not found
them to have been used as part of a repair procedure for an in-service
component, nor have they been found to have been used to specifically
target the removal of contaminants/oxides from surface-opening crevices.
The present inventors have recognized that the random action of the atoms
of the colloidal particles will function with sufficient time to allow
the cleaning solution to penetrate a surface-exposed crevice and to
assist the cleaning process within the crevice by the atomic level
movement of the particles against the entrapped contaminants/oxides.
Furthermore, the movement of particles against the contaminants/oxides
within a crevice may be enhanced by mechanical energy such as ultrasonic
energy. The improved repair procedure described herein may further
provide colloidal particles which are particularly effective for removing
contaminants/oxides which are known to be present within the crevice.

[0018] An embodiment of the invention includes a colloidal mixture or
slurry of nanoparticles in a polar solvent wherein the pH of the slurry
is maintained at about 5 to 9 and at the isoelectric point of the
nanoparticles to minimize or prevent flocculation (i.e., agglomeration)
due to attractive van der Waals forces. Another embodiment of the
invention includes a colloidal mixture or slurry of nanoparticles in a
nonpolar solution where a surfactant is added to minimize or prevent
agglomeration. The optional use of multiple nanoparticles with different
isoelectric points within a single cleaning solution also provides a
broader range of optimal cleaning. Furthermore, the properties of the
particles may be selected for a particular application, such as using a
relatively "soft" ceramic or one with less abrasive properties in a
slurry when cleaning a softer substrate. Similarly, for areas needing
more aggressive cleaning, relatively harder ceramics such as alumina and
silicon carbide can be used. A hardness of a material of the
nanoparticles may be selected to be harder than a hardness of the
contaminant material to be removed but softer than a hardness of a
material of the surface.

[0019] Another embodiment of the invention includes applying the exemplary
slurries to an article or portion thereof to be cleaned, and the distance
between nanoparticles in the slurry is maintained in an optimal physical
excitation energy state, thereby penetrating a crevice and abrading
contaminants/oxides such as iron oxide and physically removing it from a
crevice.

[0020] As used herein, a nanoparticle may be any particle defined as a
small object that behaves as a whole unit in terms of its transport and
properties and according to size, and exhibits a range between 1 and 2500
nanometers, preferably less than 100 nm, for any dimension. There are
several methods for creating nanoparticles, including both attrition and
pyrolysis, which are available in various shapes including spheres, rods,
and films. Suitable nanoparticles may be organic or inorganic, and
include ceramics, metal oxides, carbides, nitrides, metalloids and
combinations thereof. Metal oxides include crystalline solids that
contain a metal cation and an oxide anion not limited to alumina, silica,
anatase, zirconia, hematite, lead oxide, and magnesia. Nitrides may
include any of a class of chemical compounds in which nitrogen is
combined with an element of similar or lower electronegativity, such as
metals, in particular boron, vanadium, silicon, titanium, and tantalum
which are very refractory, resistant to chemical attack, and hard.
Carbides include compounds composed of carbon and a less electronegative
element and may include tungsten carbide, silicon carbide, and boron
carbide.

[0021] Suitable solvents can be either polar or non polar and may include
pentane, cyclopentane, hexane, cyclohexane, benzene, toluene, dioxane,
diethyl ether, dichloromethane, THF, ethyl acetate, acetone, DMF, MeCN,
DMSO, formic acid, butanol, isopopanol, propanol, ethanol, methanol,
acetic acid, and water. In an embodiment where the nanoparticle solution
comprises a polar solvent, the nanoparticles may be maintained at the
isoelectric point. Table 1 below illustrates ceramic materials which may
be used and the pH of the solution at the associated isoelectric point.
The isoelectric point is the value of pH at which the colloidal particle
remains stationary in an electrical field with sufficient electrostatic
repulsion between particles in order to prevent agglomeration. In this
embodiment, the Zeta potential of the nanoparticles may be at least +/-20
mV (i.e. greater than +20 mV or less than -20 mV) to achieve an optimal
physical excitation energy state, causing nanoparticles to abrade
contaminants/oxides and physically remove them from a surface or crevice.

[0022] The pH of the solution may be adjusted before or after adding
nanoparticles to form the dispersion. Suitable pH adjusters include, for
example, bases such as potassium hydroxide, ammonium hydroxide, sodium
carbonate, and mixtures thereof, as well as acids such as mineral acids
(e.g., nitric acid and sulfuric acid) and organic acids (e.g., acetic
acid, citric acid, malonic acid, succinic acid, tartaric acid, and oxalic
acid). An example of solutions that may be customized to target narrow
cracks for gas turbine engine applications may include the following
three formulas, when in polar solvent:

[0023] It is an embodiment of the invention to have a variety of
nanoparticles in different concentrations contained in the slurry with a
pH threshold of about 5 to 9. Generally, mixed oxides will exhibit
isoelectric point values intermediate to those of corresponding pure
oxides. In the instance where a concentration of various nanoparticles
exhibits ideal cleaning properties, but the pH is outside a desirable
threshold, the pH may be adjusted and surfactants may be added to
maintain the Zeta potential at least at +/-20 mV.

[0024] In an embodiment where the nanoparticle solution comprises a non
polar solvent, the inventors have discovered that the pH of the slurry is
not critical to preventing agglomeration. In this example, however, the
Zeta potential may be at least +/-20 mV such that sufficient
electrostatic repulsion exists between particles to prevent
agglomeration, which may be accomplished by the addition of surfactant.
Surfactants include dispersants (a dispersing agent or plasticizer) and
are additives that increase the plasticity or fluidity of the colloid to
improve the separation of nanoparticles and prevent agglomeration and are
not limited to non-surface active polymers or surface-active substances
from a concentration of about 0.1% to about 30% by volume. An example of
solutions that may be customized to target narrow cracks may include the
following two formulas, when in non polar solvent:

[0026] It is one embodiment of the present invention to apply a slurry to
a braze which forms a portion of a surface of a gas turbine engine
exposed to a working fluid during a post-operation service activity. The
slurry may be a mixture of different nanoparticles in suspension. For
example, combining a nitride and carbide at different concentrations, and
maintaining the mixture at its isoelectric point in a solution of water,
for example. The composition may be applied to the braze by known methods
in the art, not limited to spray, brush, or bath applications.

[0027] It is a further embodiment of the invention to provide additional
cleaning to a region in order to complement the action of the colloid
with an appropriate mechanical action, such as by applying ultrasonic
energy. A method of cleaning may include the steps of: applying a
colloidal solution to a surface; agitating the colloidal solution to
mechanically engage the nanoparticles against a contaminant material
disposed within the crevices to loosen the contaminant material from the
crevices; and removing the colloidal solution and loosened contaminant
material from the surface. A vacuum may be used to remove loosened
contaminants/oxides from the surface and from within crevices. Upon
cleaning of a surface and its surface-exposed crevices using a colloidal
solution as described above, a subsequent material overlay (braze, weld,
transient liquid phase bonding, etc.) will bond optimally with the
cleaned surface and will better fill the cleaned crevice regions than can
be achieved with prior art cleaning procedures. The improved sealing of
crevice tips achieved with the present invention will reduce or prevent
premature cracking at the crevice site that has been experienced with
prior art cleaning/repair procedures.

[0028] While various embodiments of the present invention have been shown
and described herein, it will be obvious that such embodiments are
provided by way of example only. Numerous variations, changes and
substitutions may be made without departing from the invention herein.
Accordingly, it is intended that the invention be limited only by the
spirit and scope of the appended claims.